Evidence is, in a general sense, anything used to support an assertion. The standards used to determine what sort of evidence is acceptable can vary, depending on the situation.

What may qualify as evidence in a casual argument might not qualify in a formal debate, legal proceeding or scientific investigation. Hearsay, for example, is often used in casual conversation to support a claim, but it isn't permissible in most courtrooms. In science, evidence is usually expected to be empiricallyobservable and repeatable. Additionally, if a piece of evidence implicates more than one cause, additional evidence should be presented to exclude the other possibilities. Scientific experimentation depends on the analysis of many observations in order to determine consistent patterns and to reduce elements of chance or uncertainty. Any claims, or hypotheses, derived from observations must be falsifiable if disconfirming evidence is observed.

Below is a list of the standards of evidence in science, as well as why each standard is important. Each qualification for evidence is meant to minimize error and maximize accuracy, so the evidence can be as useful as possible.

Without a proposed and demonstrated mechanism between an asserted cause and effect, there's no way to demonstrate that it's anything more than a coincidence. The causal link between flipping a light switch and the light coming on is that the flow of electricity is allowed to the light bulb, which lights up. This causal link can be easily demonstrated. Conversely, what is the causal link between burying a statue of St. Anthony upside down by the "for sale" sign for the house that's on the market, and the house selling faster, and how would one demonstrate this causal link? Without the causal link, it's a form of post hoc ergo popter hoc to assert that they're connected.

Repeatability

It's possible that when event B follows A that it was simply a fluke. Being able to repeatedly and consistently demonstrate A -> B ensures that statistical noise can be eliminated from the investigation.

Falsifiability

It's possible to construct claims that cannot be disproven, yet are, in actuality, untrue. Prayer is a good example of this. The model of prayer is constructed in a way that, no matter the outcome, prayer has always succeeded. Such claims tend to be indistinguishable from them not occurring at all, and aren't useful for explaining anything, as their demonstrability is sacrificed to make them unfalsifiable. What is the utility of asserting something that cannot be distinguished from it not happening at all?

Testability

Just in case a person making a claim isn't simply lying or incorrect for some reason, testability gives us the ability to verify each others' claims. Until a claim is tested, it's simply an assertion. How does one test the assertion that the birds and trees are evidence for creatoin? Not everything has to be directly observable or testable. Like magnetism that we cannot detect with any senses, we can investigate it indirectly, with metal objects for instance. Evolution makes testable claims. It can be used to predict that at one point, water creatures evolved into land creatures, and we could potentially find transitional forms of when some did evolve. This was successfully tested when we discovered Tiktaalik, as predicted.

Presentability

Evidence must be presentable to others, otherwise, one can claim that one has incontrovertible evidence, but it's currently unavailable. Without presentability, there's no way to validate the evidence asserted. Personal anecdotes often fail in this category, as they are necessarily first-person, and can only be asserted to others.

Objective

The mind is so prone to error, misinterpretation and bias, that any data that originates from a mind is necessarily extremely unreliable. This is why testimonial evidence ranks so low in criminal trials. When someone claims a god exists because he/she can feel him, we cannot distinguish between the person's assertion being correct, and the person being schizophrenic. Data that originates from objective reality has this subjective error greatly minimized. Popular in the Intelligent design movement is the asserted evidence, "We can tell when someone appears to be designed, so when we look at nature, and it appears to be designed, it therefore is an indicated that it in fact designed." That's a matter of subjective opinion. It's like saying "We can recognize a cube when we see it, and if an object appears to be a cube, then we have established that it is a cube." Unlike "the appearance of design", we have an objective definition of a cube that can be objectively measured and confirmed.

Exclusion

Exclusion has more to do with corroborating evidence than the specific evidence in question, and is frequently where apologetics fail. If one is investigating whether Bob is a smoker, and discovers that Bob's house burned down in the past, that could indicate that he was smoking in bed, had an accident, and the house caught on fire. The question arises, how do know the fire wasn't cause by other possibilities, such as a lightning strike, bad wiring, or knocked over candle? Further evidence excludes one possibility from the others, until it's been sufficiently demonstrated that the asserted claim is the correct possibility. Likewise, when apologists cite the existence of Earth as evidence for a Creator, the question arises, what evidence do we have that it was a god, and not some other natural cause that we aren't aware of yet? The existence of an effect doesn't prove the asserted cause.

Counter-apologetics

When skeptics object to apologist's claims on the basis of insufficient evidence, they're generally referring to a lack of scientific evidence or a preponderance of anecdotal evidence. For any given claim, there exists a burden of proof which must be supported by evidence. The defining characteristic of the required evidence rests in its quality, not quantity.

If, for example, someone claims "God answered my prayer" that is both an assertion (of a specific occurrence) and evidence for the general assertions about the existence of God and the efficacy of prayer. Unfortunately, the quality of that evidence is exceptionally low. So low, in fact, that such a claim is only considered evidence in the very broad, definitional sense. Anecdotal evidence and bald assertions have no evidentiary value and serve only as additional claims.

Suppose that 2.5 million people all testified, "God answered my prayer". The sheer quantity of evidence is sufficient to warrant an investigation of the claims, but the testimonies still don't qualify as sufficient evidence to support acceptance of the claim. Increasing the quantity of claims doesn't increase the quality.

Apologists commonly make claims supported by anecdotal evidence, hoping that the sheer quantity will be sufficiently convincing. Unfortunately, this tactic is often effective and claims like, "80% of the population believes in God — they can't all be wrong" can be very convincing to those who don't critically examine the claim. This argument works because people are reluctant to write off that 80% as delusional, stupid or insane. (See Argumentum ad populum.)

The truth is that they most definitely can be wrong. At some point in human history, the overwhelming majority believed that the Earth was the center of the universe. That majority was wrong. They weren't necessarily delusional, stupid or insane; they were simply ignorant of the facts, and wrong.

Frequently, atheists are accused of being "in denial about all the evidence around us". It's important to point out that atheists aren't in denial; the evidence has been rejected for not meeting the standards of evidence, like a potential employee might not qualify for a job and is thus rejected. Rejecting that person's job application is not the employer being "in denial" about how qualified he/she is. The standards can be clearly and lucidly laid out for an apologist, including why the standards are important. At this point, the apologist typically abandons the concepts of logic and reason altogether.